The present invention relates to a device for supplying seal air to bearings of a gas turbine engine.
In a multiple-shaft bypass jet engine, an inner shaft supporting the rotors of a low pressure compressor and a low pressure turbine and an outer shaft supporting the rotors of a high pressure compressor and a high pressure turbine typically consist of hollow shafts which are coaxially nested with each other. The outer and inner shafts are supported by separate bearings at their front and rear ends, and each bearing is lubricated by a forced lubrication system which blows lubricating oil fed by a pump to the bearing.
In such a forced lubricating system, to positively prevent the lubricating oil from leaking out of the bearing boxes provided in the front and rear ends of the outer and inner shafts, high pressure air drawn from one of the compressors is conducted to the exterior of the oil seals of the bearing boxes to keep the interior of the bearing boxes at a lower pressure than the supporting part.
The high pressure air which is supplied to the exterior of the bearing boxes may consist of the high pressure air produced by the high pressure compressor to be used in the combustion chambers, the passages for the seal air, in particular for the front bearing box, tends to be highly complex because of the presence of an intake duct in front of the high pressure compressor.
To deal with such a problem, it has been proposed to provide an outlet to each of an intermediate part of the impeller casing of the centrifugal compressor and an outer peripheral part of a back side of the centrifugal compressor so as to provide separate seal air supply passages for the seal portions of the front and rear gear boxes and minimize the length of the seal air passages.
However, because the air pressure from the centrifugal compressor varies depending on the point of drawing the high pressure air, supplying seal air to the seal portions via separate passages and from different points of drawing the high pressure air causes an evenness in the seal pressure between the front and rear gear boxes. Therefore, in case of a failure of a mechanical seal, the lubricating oil may be blown toward the part of the lower seal pressure.
In view of such problems of the prior art, a primary object of the present invention is to provide a device for supplying seal air to the bearings of a gas turbine engine which allows the seal pressure to be kept even between the front and rear gear boxes without complicating the passages for supplying the seal air to the bearing boxes.
A second object of the present invention is to provide a device for supplying seal air to the bearings of a gas turbine engine which would prevent any serious loss of lubricating oil even when one of the seals should fail.
A third object of the present invention is to provide a device for supplying seal air to the bearings of a gas turbine engine which would not complicate the structure of the engine.
According to the present invention, such objects can be accomplished by providing a device for supplying seal air to bearing boxes of a gas turbine engine, comprising: an inner shaft connected to a low pressure compressor and a low pressure turbine; an outer shaft, coaxially disposed with respect to the inner shaft, connected to a high pressure compressor and a high pressure turbine; a front and a rear inner shaft bearings supporting a front and a rear end of the inner shaft, respectively; a front and a rear outer shaft bearings supporting a front and a rear end of the inner shaft, respectively; a front bearing box substantially enclosing the front inner shaft and front outer shaft bearings, the front bearing box being separated into an outer chamber exposed to a front end of the front inner bearing and a rear end of the front outer bearing, and an inner chamber exposed to a rear end of the front inner bearing and a front end of the front outer bearing; a rear bearing box substantially enclosing the rear outer shaft and rear inner shaft bearings, the rear bearing box being separated into an outer chamber exposed to a front end of the rear outer bearing and a rear end of the rear inner bearing, and an inner chamber exposed to a rear end of the rear outer bearing and a front end of the rear inner bearing; a hollow connecting shaft extending coaxially between rotors of the high pressure compressor and the high pressure turbine; a first passage having a first end communicated with a part of the high pressure compressor and a second end communicated with an annular gap defined between the connecting shaft and outer shaft; a second passage extending along an outer periphery of the outer shaft and having a first end communicated with the annular gap and a second end communicated with the outer chamber of the front bearing box; and a third passage extending along an outer periphery of the outer shaft and having a first end communicated with the annular gap and a second end communicated with the outer chamber of the rear bearing box.
Thus, the seal air drawn from a single point of the compressor can be supplied to both the front and rear bearing boxes evenly without complicating the passages. The inner chamber of each of the bearing boxes typically communicates with an annular gap between the inner and outer shafts, and is appropriately drained or vented out of the inner chamber after separating oil from the air flow.
In particular, if the second and third passages each include a part having a narrow cross section than that of the first passage, the seal pressure between the two seal portions can be made highly uniform. Furthermore, because of the flow restricting action of such narrowed parts, even if one of the seals is damaged, because the seal pressure drops only in the affected bearing box, a concentrated oil leakage from the damaged seal can be avoided while the seal pressure in the undamaged bearing box is maintained.
Also, if the second end of the first passage comprises a circumferentially elongated slot formed in the connecting shaft for introducing the high pressure air to the inner periphery of the rotor of the high pressure compressor, the stress acting on the peripheral part of the hole due to the centrifugal force can be reduced, and the admission of the swirling flow due to the rotation of the rotor of the compressor into the first passage is favorably promoted.